Induction heating is to produce heat in such a manner that a magnetic field is generated by the passage of currents through heating coils to generate an overcurrent in a member to be heated, and it is adopted in various fields since it can generate a high temperature which cannot be obtained by resistance heating. FIG. 8 schematically shows the outline of an induction heating unit which hardens a roll of a rolling mill and so on.
In FIG. 8, a roll 10 is composed of a roll body 12 and journals 14 disposed at both ends thereof. When the roll 10 is to be hardened by the induction heating, a heating coil 16 which generates a magnetic field with a high magnetic flux density and a temperature keeping coil 18 which generates a magnetic field with a magnetic flux density lower than this are provided in an induction heating unit 15 and they are connected respectively to high-frequency power supplies 20, 22 constituted of corresponding inverters. These heating coil 16 and temperature keeping coil 18 are disposed adjacent to each other without any space being made therebetween, thereby preventing temperature decrease at a border portion between both of the coils 16, 18. In order to harden the roll 10, the roll 10 is moved forward toward the coils 16, 18 in a direction of an arrow 24 and a surface layer portion of the roll body 12 is heated at about 950° C.
FIG. 9 shows the outline of a partial electromagnetic induction heating unit. In this partial electromagnetic induction heating unit 30, a plurality of heating coils 32 (32a to 32c) are arranged coaxially in a vertical direction and connected respectively to high-frequency power supplies 34 (34a to 34c) constituted of corresponding inverters. For example, an end (lower end) of a carbon rod 36 is inserted into the heating coils 32, gas is supplied to the periphery of the carbon rod 36 to heat it at about 1500° C. by the heating coil 32, and the gas is caused to react to this. In this case, since the heat escapes upward, power supplies 34 are controlled so as to make a magnetic flux density become higher toward an upper one of the heating coils 32. Furthermore, the heating coils 32 are arranged adjacent to each other in order to prevent temperature decrease in border potions.
FIG. 10 shows the outline of a unit for heating a container by electromagnetic induction. In this induction heating unit 44, powdered silicon carbide (SiC) 42 is put inside a crucible 40 made of, for example, carbon, this is heated by heating coils 48 (48a, 48b), and the silicon carbide 42 is evaporated to be deposited in a work 46. The induction heating unit 44 includes the two heating coils 48a, 48b disposed coaxially in a vertical direction, which are connected respectively to high-frequency power supplies 50 (50a, 50b) constituted of inverters, and the heating coil 48b on a lower side generates a magnetic field with a high magnetic flux density to heat the silicon carbide 42.
FIG. 11 shows the outline of a so-called Baumkuchen-type induction heating unit. This induction heating unit 60 includes a doughnut-shaped stage 62 made of carbon or the like and a plurality of semiconductor wafers 64 are to be disposed on an upper surface of the stage 62. Heating coils 66 are disposed under the stage 62 so that the semiconductor wafers 64 can be heated by the passage of electricity through the heating coil 66. Furthermore, the heating coils 66 consist of an outer coil 66a, a center coil 66b, and an inner coil 66c, which are connected respectively to high-frequency power supplies 68 (68a to 68c) constituted of corresponding inverters so that the entire stage 62 can be uniformly heated. In this case, the coils 66a to 66c are also disposed adjacent to each other so as to be in contact with each other, thereby preventing temperature decrease in border portions of the coils.
FIG. 12 shows the outline of an induction heating unit for extrusion forming. This induction heating unit 70 includes a plurality of heating coils 72 (72a to 72c) arranged coaxially in a horizontal direction, which are connected respectively to high-frequency power supplies 74 (74a to 74c) constituted of corresponding inverters, and a metal material 76 placed inside the heating coils 72 is heated in such a manner that the temperature decreases from a front end portion in the workpiece toward a rear end portion in the workpiece. The heating coils 72a to 72c are disposed adjacent to each other to prevent temperature decrease in border portions. A similar induction heating unit is also used in a case of SSF (Semi Solid Forging) in which a metal material is forged in the state where a liquid phase and a solid phase coexist.
Since a high power efficiency can be obtained in induction heating, it is often performed by a so-called resonance-type inverter having a resonance circuit. Further, in the induction heating units having the plural heating coils as described above, the coils are disposed adjacent to each other in order to prevent the temperature decrease in the border portions of the respective heating coils. Consequently, mutual induction occurs among the plural heating coils since a magnetic flux generated by one of the heating coils influences the other heating coils. Therefore, in the induction heating unit including the heating coils corresponding to a plurality of inverters, since the state of the mutual induction among the heating coils changes due to load fluctuation and so on, distortion occurs in the current (heating coil current) in each of the heating coils and a phase deviation occurs between the heating coil currents. Consequently, in the induction heating unit including the heating coils corresponding to the plural inverters, unless the frequencies of the respective load currents are equalized and the phases of the respective heating coil currents are fixedly maintained, a highly precise control of a heating temperature becomes difficult and the temperature decrease in the border portions of the heating coils is caused.
Therefore, a method of preventing the occurrence of the adverse effect of the mutual induction has been proposed in which magnetic force shielding coils are inserted between heating coils and they absorb magnetic fluxes in end portions of the heating coils. It is also proposed that two heating coils are connected in parallel to one frequency converter (high-frequency inverter), a variable reactor is connected to one of the heating coils in series, and the variable reactor is adjusted by an L cycle to vary a voltage value (Japanese Utility Model Publication No. Hei 3-39482).
The method described above in which the magnetic force shielding coils are disposed in the border portions of the heating coils, however, cannot achieve uniform heating since the magnetic fluxes in the end portions of the coils are absorbed by the magnetic force shielding coils to cause the temperature decrease in these portions. The method in which the variable reactor is connected in series to one of the heating coils to vary a voltage by the variable reactor as described in Japanese Utility Model Publication No. 3-39482 also has such disadvantages that controlling the variable reactor changes the entire frequency, a time constant of power control is long, the power control of one unit changes a power value of each of the heating coils of the entire system so that it is difficult to independently control temperature for each of the heating coils, and so on.
Meanwhile, in each of the inverters, inverter output efficiency (power factor) becomes low unless a phase difference between its output current and output voltage is made small so that capacity decrease and efficiency degradation of the inverter are caused. Therefore, it is preferable that the inverter is operated in such a manner that its output current and output voltage are synchronized with each other.
The present invention is made to solve the disadvantages of the aforesaid prior arts and it is an object of the present invention to prevent the temperature decrease in the border portions of the heating coils and to enable the elimination of the influence caused by the mutual induction.
It is another object of the present invention to prevent the change in the state of the mutual induction.
It is still another object of the present invention to enable improvement in the power factor of the inverter.